Rework server to use experimental mutable tower-lsp branch

[JoshuaBatty]

Description

This PR uses the experimental branch in tower-lsp that allows notification methods to take &mut self. This means we no longer need to wrap all of our types in RwLock's as we have clear boundaries where we can mutate state owned by the server and where we just take a reference.

Not looking to get this PR merged at this stage, just opening for visibility. I must say, it really does feel soooo much nicer than what we currently are doing. I'm going to reach out to @ebkalderon to see whats left to do before this branch can be merged into master in tower-lsp.

See this issue for more context on the API change in tower-lsp

Checklist

• I have linked to any relevant issues.
• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation where relevant (API docs, the reference, and the Sway book).
• I have added tests that prove my fix is effective or that my feature works.
• I have added (or requested a maintainer to add) the necessary Breaking* or New Feature labels where relevant.
• I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
• I have requested a review from the relevant team or maintainers.

[JoshuaBatty]

Closing as it doesn't seem like there is going to be any movement on getting this support merged in the master branch of tower-lsp. We have since optimised the server to work with references so most of the performance wins are already evident.